Early hydrocarbon production occurred mostly from “non-source” reservoirs. That is, most hydrocarbon production has been from porous underground formations which trapped hydrocarbons that migrated from other locations. Because of the porosity of the formations in which hydrocarbons accumulate, the primary goal of drilling was merely to locate a wellbore at a suitable location within the porous underground formation. To the extent the reservoir engineer or driller considered other wellbores during the planning and/or drilling stages, the consideration was only that two wellbores not collide or intersect each other.
A large portion of more recent hydrocarbon production comes from “source” reservoirs, sometimes referred to as shale reservoirs or just shales. Shales are considered source reservoirs because the shales are the physical location where organic material was converted to hydrocarbons in the distant past based on high pressure and high heat. As the conversion from organic material to hydrocarbons took place, expansion occurred which thus forced much of the oil and gas produced to migrate to other “non-source” reservoir locations. However, shales still contain significant residual quantities of hydrocarbons.
Unlike porous non-source reservoirs, however, shales have very low permeability. That is, though there may be significant porosity per unit volume of shale, the ability of hydrocarbons to move through the pore spaces has become limited over time. Based in part on the low permeability of shales, the number of wellbores drilled into a particular shale formation may be greater to achieve adequate drainage than would be required for a highly permeable “non-source” reservoir.